The present invention relates to a bone transmitting hearing aid/bone transmitting vibrator for generating or monitoring vibrations in accordance with the variable reluctance principle comprising a coil for generating/monitoring a magnetic signal flux, a bobbin body of a magnetic conductive material, one or more yokes of magnetic conductive material, and one or more permanent magnets for generating a magnetic biassing flux.
Bone transmitting hearing aids are used by patients who can not use conventional air transmitting hearing aids e.g., due to chronic middle ear disease or a congenital/acquired deformity. A traditional bone transmitting hearing aid consists of a bone transmitting vibrator enclosed in a polymer shell which is pressed with a constant pressure of 3-5 Newton against the skin over the bone behind the ear. Microphone, amplifier, and current source are placed in their own enclosure at a suitable site and at a secure distance from the vibrator to avoid feed-back coupling problems. The most essential drawbacks of this type of bone transmitting hearing aids is that it is uncomfortable to wear due to the constant pressure and that the soft skin over the bone deteriorate the transmission of vibrations to the bone.
Since the beginning of the 1980""s there is a new type bone transmitting devicexe2x80x94a bone anchored hearing aid (BAHA)xe2x80x94where the bone transmitting vibrator is connected directly to the bone via a skin penetrating and bone anchored implant of titanium, cf e.g., SE-A-81 07 161-5, SE-A-94 04 188-6 or Tjellstrxc3x6m and Hxc3xa5kansson, The Bone Anchored Hearing Aidxe2x80x94Design principles, indications, and long-term clinical results, Otolayngol. Clin. N. Am. Vol. 28, No. 1, (1995). In this way a bone transmitting hearing aid is obtained which provides for higher amplification, splendid carrying comfort, and where all parts can be enclosed in the same housing. In a future solution the vibrator can be implanted completely and thereby skin and soft tissue can remain intact. Signal and necessary energy can in this case be transferred through intact skin by means of inductive connection. At more severe hearing damages where the energy demand is large the energy can be transferred by means of skin penetrating (percutaneous) electric connection device, cf e.g., SE 9704752-6. The advantages implanting the whole vibrator into the temporal bone compared with a vibrator being externally situated is, besides the pure medical ones, that an increased sensitivity is obtained, the size of the externally placed unit becomes smaller and stability margins becomes improved.
It is of course of utmost importance that BAHA vibrators in general and implantable ones in particular are (1) efficient, to keep current consumption down, (2) small, in order to be able to be placed in the temporal bone, and (3) reliable, as a repair/exchange of the vibrator requires a surgical incision. The need to improve conventional bone transmitting hearing aids in the above mentioned respects is perhaps the most important motif behind the present invention.
Another area of application where the bone transmitting vibrator is used is within clinical audiometry. At a conventional audiometry examination both air transmitting and bone transmitting threshold values are regularly determined The vibrator used at bone transmitting audiometry is of the same kind as used for bone transmitting hearing aids with the difference that the audiometry vibrator shall be capable of determining bone thresholds down to 250 Hz. It is commonly known the vibrators of today to be used in audiometry, e.g., B71 from Radio Ear, shows dissatisfactory high distorsion at low frequencies due to an intrinsic problem of this construction. Thus even here there is a great demand for improving the technology.
Vibrators based on piezo electricity, magnetostriction (magnetic elongation), and electromagnetism of the moving coil type are not used in bone transmitting hearing aids or audiometry vibrators mainly due to bad response at low frequencies. The devices used are electromagnetic vibrators of the variable reluctance type.
Prior Art
A cross-section of a conventional (State of the Art) vibrator of variable reluctance type of hitherto known type is shown in FIG. 1. The vibrator of FIG. 1 is substantially circularly symmetric. It consists of on one hand an annular permanent magnet, a coil coiled around an annular bobbin body, as well, and a counter mass connected in a suitable manner to a rigid unit (lower part), and on the other hand of a vibrator plate connected with a spring elemnt and a suitable adapter for connection to the load (top part). The bobin body and the vibrator plate are made of magnetic field well conductive material, suitably special treated soft iron. The vibrator plate functions as a yoke closing both the static (biassing) magnetic flux "PHgr"0 generated by the permanent magnet and the signal flux "PHgr"_ generated by a signal current flowing through the coil. The total development of force in the air gap is determined under certain presumptions approximatively by
Ftotxe2x88x9d("PHgr"0+"PHgr")2="PHgr"02+2."PHgr"."PHgr"0+"PHgr"2xe2x80x83xe2x80x83Equ. 1
The term "PHgr"02 represents the static force of the permanent magnet, the term 2."PHgr". "PHgr"0 represnts the useful signal flux and the term "PHgr"2 represents a non-desired distortion. The primary task of the counter mass is to add mass to obtain a suitable resonance frequency fr according to the relation
frxe2x88x9d1/m.c Hzxe2x80x83xe2x80x83Equ. 2
Wherein m is the mass of the lower part of the vibrator (including the outer rigid part of the spring element) and c is the compliance (resilience) of the spring element. In the following the mass m is called the counter holding unit. The resonance frequency may, e.g., in accordance with Equ. 2 be lowered by increasing the weight of the counter holding unit (m) or increasing the compliance of the spring element (c).
Drawbacks at Conventional Variable Reluctance Vibrator
Of the above description it is evident that a conventional variable relucatance vibrator is construed in such a way that the magnetic signal flux coincides/follows the static flux (biassing flux) along is entire run, cf FIG. 1. It leads to the fact that the properties of the electrodynamic change deteriorate as permanent magnets as a rule have a low relative permeability r (low permeability provides for a high magnetic flux resistance, which decreases the generation of signal force).
In order to prevent that the air gap which is created between the vibrator plate and the lower part of the vibrator does not collapse due to the static force (xcexc"PHgr"02) a spring element is required that keeps the parts apart. This spring element consists normally of a plate spring package with or without dampening coating as described in SE-A-85 02426-3. In resting condition which corresponds to an air gap of 50 to 100 nm the spring is so bent out to such a degree that its returning force exactly balances the attracting force of the permanent magnet. The Attraction force of the permanent magnet thus all the time strives to reduce the air gap created by balancing the magnet force and the spring force. Ageing of the spring as well as outer mechanical strains may thus lead to that the air gap of the vibrator collapses. If this should occur the sound of the vibrator becomes strongly distorted and the vibrator has to be repaired.
Another problem is that this type of vibrator, at higher signal levels, creates a high harmonic distortion of the second order due to the term "PHgr"xe2x80x942, cf Equ. 1. In order to obtain a good linearity a high biassing flux ("PHgr"0) is required, which requires high stiffness of the returning spring which in turn leads to a higher resonance frequency. This increase of the resonance frequency can be counteracted by increasing the counter holding mass (cf Equ 2) but at the price of increased weight and size.
Balanced Armature
In order both to be able to maintain a high biassing flux and simultaneously to use a softer returning spring one has, since long, used a so called Abalanced armature@-principle, cf e.g., U.S. Pat. No. 3,491,436 by Elmer V. Carlsson when constructing small loud speakers for placement in the auditory meatus (generation of air spread sound). The thin arm (Aarmature reedxe2x80x9d) of soft iron material which also functions as returning spring, is placed between two permanent magnets. In the middle position the magnets draw equally much and the arm is situated in a balanced position. The demand on the spring constant of the returning spring will hereby become much smaller than if a magnet circuit of the traditional type was to be used. These transducers are adapted to drive a light weight membrane for air borne creation of sound and the construction can not be transferred to a bone transmitting vibrator the load and working conditions (i.e. the skull bone) differ considerably from air. Further, the signal flux is hereby not only lead through the soft iron material and the air gap but also through the permanent magnet material which as a rule possesses a high reluctance (magnetic flux resistance) relative to the soft iron material.
The proposed invention is a new vibrator of variable relectance type which is characterized in that the signal flux is closed through the bobbin body and yoke as well as by two or more common air gaps where biassing flux and signal flux cooperates for generating the signal force. Both the bobbin body and yoke are made of material which leads magnetism very well, such as e.g., specially prepared soft iron material. The permanent magnets generating biassing flux can be placed in many different ways under the condition that the biassing flux in each embodiment is led in such a way that it cooperates with the signal flux in the air gaps for generating the signal force in accordance with Equ. 1. Different from known technology the signal flux, in the proposed solution herein, is closed through the soft iron material and air gaps without passing the permanent magnet(s). One advantage using this solution is that the efficiency of the vibrator is improved as the permanent magnets, as mentioned above, in general have bad dynamic (signal providing) properties compared with the soft iron material.
Another advantage is that the static flux cooperates in the air gaps according to the principle of Abalanced armature@ so that the static forces eliminates each other. This means that the vibrator can be made smaller for a given resonance frequency as the returning spring (the spring element) can be softer as it need not counteract any static force in the neutral position and the counter mass can thus be lighter, i.e., smaller to a corresponding degree.
Another advantage using the balanced armature is that even the quadratic distortion terms will be outbalanced. Most of all it is due for the second harmonic overtone which otherwise can be very dominating at low frequencies, which is particularly annoying at audiometry vibrators.
The return to neutral position is secured by one or more spring elements. The spring elements can e.g., consist of plate springs with or without dampening coating. Furthermore the air gap can be provided small elastic pillows to provide for a progressive resiliency which also provides a soft restriction (compression) of high sounds. The pillows in the air gap counteracts the possibility to air gap collapse as well.
In the two embodiments of the invention described in detail below, the coil and the permanent magnet(s), as different from known technology, been split in a new way which has been made possible due to the balanced hanging. In one unit the generation of the magnetic signal (coil and bobbin body) flux is carried out, and in the other unit the generation of the magnetic biassing flux (permanent magnet(s) and yoke) is carried out. The spring element connects the two units while observing, as described above, that it is formed two or more air gaps between the units where the static forces are outbalanced and where magnet bias and signal flux cooperate for generating the signal force. The advantage splitting the units in this way is that vibrational stress on the thin connecting lines to the coil become minimal as the coil via the bobbin body is connected to the skull bone which has a very high mechanical impedance (Hakansson et al, The mechanical point impedance of the human head, with and without skin penetration, J. Acoust. Soc. Am., Vol. 80, No. 4, October 1986). At resonance frequency the counteracting unit will swing with relatively large amplitudes while the coil moves relatively little and transfers, mainly, forces only. High reliability when it comes to the durability of the lines of the coil will be of utmost importance when the vibrator is implanted.
The application of the invention is not restricted to bone transmitting hearing aids but can, with advantage, also be used as audio metry vibrator and other loudspeaker applications as well as vibration provider.